1. Field of the Invention
This invention relates to transistor detecting circuits and particularly to a circuit with switching means connected between the base and collector of the transistor to improve linearity of operation of the detecting circuit over a wide range of signal amplitudes.
2. Description of the Prior Art
Conventional transistorized detecting circuits consist, basically, of a detecting transistor having a base to which the input signal is applied by way of a capacitor and an emitter from which the rectified signal is obtained. In addition, a rectifying diode is connected to the base of the detecting transistor and has a polarity opposite to that of the base-emitter junction of the transistor. The other terminal of the diode is connected to a biasing source in the form of a second diode to add a voltage that corresponds to the forward voltage drop across the rectifying diode. The resulting voltage-doubled, rectified signal is obtained at the emitter output terminal of the circuit.
One disadvantage of the prior art detecting circuit is that, if a small signal voltage having a magnitude below the base-emitter voltage V.sub.be of the detecting transistor is applied to the input terminal of the circuit, the signal will be severely distorted. This is due to the fact that the threshold level of the diode cannot be selected to be equal to that of the detecting transistor. Accordingly, the dynamic range of amplitudes of signals from such a detecting circuit is very narrow, about 20 to 25db at most.
It is one of the objects of the present invention to provide an improved detecting circuit in which a transistor is utilized as the rectifying element and is so connected as to provide greater linearity of operation than transistor detecting circuits of the prior art.
It is another object of this invention to provide a detecting circuit having a wider dynamic range than was possible with previous detecting circuits.
A still further object of this invention is to provide a detecting circuit in which even signals of small amplitude may be detected substantially without distortion.
Other objects and advantages of the present invention will become clear from the detailed description of the preferred embodiments as illustrated with reference to the drawings.